A typical gas turbine used in, for example, a power plant sprays fuel into air compressed by a compressor so as to combust the air, and introduces the resultant high-temperature high-pressure combustion gas to a turbine so as to extract the output power. FIG. 11 illustrates the basic configuration of such a gas turbine. A gas turbine 100 includes a compressor 102, a combustor 103, and a turbine 101. The combustor 103 is supplied with air compressed by the compressor 102 and fuel gas flow-controlled by a fuel flow control valve 105, the degree of opening of which is controlled in accordance with the load. High-temperature combustion gas combusted in the combustor 103 is supplied to the turbine 101 and is expanded so as to drive the turbine 101. This driving force is transmitted to a power generator 150 where electricity is generated, and is also transmitted to the compressor 102 so as to drive the compressor.
In the case of a single-shaft combined-cycle power plant, rotation shafts of the gas turbine 100, the power generator 150, and a steam turbine 160 are combined into a single unit.
An inlet guide vane (IGV) 104 is provided in front of the first-stage airfoil of the compressor 102. By controlling the degree of opening of a guide vane at the inlet of the compressor, the inlet guide vane 104 changes the amount of air flowing between rotor blades of the compressor 102 and into the combustor 103 so as to control the exhaust gas temperature in the gas turbine 100 to a target value. Intake air is given a certain circumferential speed by the inlet guide vane 104 and is introduced to the compressor 102. In the compressor 102, the air introduced thereto travels through multiple stages of rotor blades and stator vanes and increases in pressure by being given energy.
In the inlet guide vane 104, a large number of movable vanes provided in the circumferential direction are supported in a rotatable manner, and an actuator is actuated in accordance with a drive signal from an operation control device 110 so that these movable vanes are moved, whereby the intake-air flow and the combustion temperature are adjusted.
More specifically, the operation control device 110 has the configuration shown in FIG. 12 for generating an IGV degree-of-opening command for the actuator of the inlet guide vane 104. The operation control device 110 has a multiplier 11, a table function unit (FX1) 12, a limiter 13, a correction function unit (FX2) 14, and a limiter function unit (FX3) 15. Basically, the degree of opening of the IGV is set on the basis of the function shown in FIG. 13(a) in accordance with a power-generator output (GT output). The correction function unit (FX2) 14 generates a GT-output correction coefficient K2 on the basis of a relationship corresponding to the compressor inlet temperature shown in FIG. 13(b), and the multiplier 11 multiplies this correction coefficient K2 by the GT output, so that a GT output value is corrected with reference to a table function. Furthermore, the limiter function unit (FX3) 15 generates an IGV maximum degree of opening M1 on the basis of a relationship corresponding to the compressor inlet temperature shown in FIG. 13(c), and the limiter 13 performs limitation so that the degree of opening of the IGV generated at the table function unit (FX1) 12 does not exceed the IGV maximum degree of opening M1.
Because the rotation shaft of the turbine 101 and the power generator 150 are linked with each other in the configuration shown in FIG. 11, the load in the power generating unit changes in accordance with a change in the system frequency. For example, in a case where the system frequency decreases, the rotation speed also decreases. In order to maintain a specific rotation speed, it is necessary to increase the amount of fuel to be supplied in the gas-turbine power generating unit. For example, Patent Literatures 1 and 2 each disclose a prior-art feature in which operation control is performed in correspondence with such a frequency change. Patent Literature 1 discloses a feature for switching to a control mode that is different from a normal control mode and that mainly focuses on recovery of the system frequency when an abnormality is detected in the system frequency. Patent Literature 2 discloses a governor-free control method in which the rate of change of the system frequency is controlled to be within a limitation range.